What is the ruling on the prayer of one who completed the Tashahhud and the Imam prostrated for forgetfulness, so the worshiper prostrated with him and then greeted after him?

You should have followed the Imam and prostrated with him for the forgetfulness (sujud (definition) al-sahw), and not delayed in greeting (saying tasleem). Indeed, the follower (ma'mum) follows his Imam and says tasleem immediately after the Imam finishes, once the obligatory part of the tashahhud (definition) and sending prayers upon the Prophet (peace be upon him) is completed. One should not occupy oneself with a sunnah (definition) act. Your prayer is valid and not nullified by this delay, but in the future, you must adhere to following the Imam.